1. Field of the Invention
The present invention relates to a hybrid integrated circuit substrate anda method of manufacturing the same, and more particularly, it relates to a low-cost and highly reliable hybrid integated circuit substrate and a manufacturing method thereof.
2. Description of the Prior Art
Generally, as circuit substrates of the above type, there have hitherto ben used a thick film hybrid integrated circuit sustrate formed by utilizing a thick film forming process in which thick film conductor paste or thick film resistive paste is printed and fired on an insulating heat-resistant substrate such as an alumina ceramic substrate, a tin film hybrid integrated circuit substrate formed by utilizing a thin film forming process such as vapor deposition and spattering, a combined hybrid integrated circuit subtrate in which a thick film and a thin film are combined and another type of hybrid integrated circuit substrate in which polymer printing resistors are formed on, e.g., a paper phenol substrate.
The thin film hybrid integrated circuit substrate is used in the field requiring high reliability, high efficiency and fine line patterns mainly for industrial electronic devices, and is costly in comparison with the thick film hybrid integrated circuit substrate.
On the other hand, the thick film hybrid integrated circuit substrate is formed by printing and firing, utilizing paste of noble metals such as silver-palladium, platinum-palladium or gold in conductor layers and cermet resistive paste such as ruthenium oxide in the resistors, to be used in most of public and industrial fields.
Further, a thick film resin circuit substrate formed by printing polymer printing resistors on an organic substrate, such as paper phenol and curing the same to be combined with conductor patterns prepared by etching of a copper foil or printing and curing of silver polymer paste and so on is used in the public field mainly for the purpose of saving cost therefor. However, such a thick film resin circuit substrate is inferior in efficiency and reliability to those having cermet resistors and thin film resistors fired at a high temperature, since it requires resistors of a type cured at a low temperature. Further, this type of circuit substrate is inferior in thermal radiation and thus cannot be used for a power supplier, and, besides, it is hard to attain a high density. Therefore, application thereof is inevitably limited to the public devices whose essential object is to save the cost therefor.
In the aforementioned thick film hybrid integrated circuit substrate, the range of the resistance value can be widened by utilizing cermet resistors of, e.g., ruthenium oxide. Further, this type of substrate is highly efficient and highly reliable since it is fired at a high temperature over 500.degree. C., and is used in most of the public and industrial fields.
However, the thick film hybrid integrated circuit substrate is costly since a noble metal is used for the conductor layers. Further, there is such a disadvantage that silver mainly used for the conductor layers is melted in molten solder when circuit elements are soldered to the conductor layers. Therefore, utilization of solder containing silver is required while the soldering process must be carefully controlled with respect to, e.g., temperature control.
In substitution for the above noble metal paste, base metal paste of, e.g., copper and nickel may be used for the conductor layers of the thick film hybrid integrated circuit substrate to reduce the cost and lower the impedance of the conductor layers. In this case, however, the substrate must be fired in a nitrogen atmosphere, and development of applicable resistive paste is required.
The descriptions which is of interest to the invention are made in the following documents. Japanese Laying-Open Gazette No. 93296/1983, "Method of Manufacturing Maltilayer Wiring Substrate" describes that a Ti layer of 1000.ANG. in thickness and a Cu layer of 5000.ANG. in thickness are formed on polyimide by spattering and then a Cu pattern plating layer of 6 .mu.m in thickness is formed by using a resist, under the heat-resisting temperature of polyimide. "Polyimide Dielectric on Hybrid Maltilayer Circuits" by John Shurboff, Motorola Inc., 1983 IEEE describes a tendency of application of polyimide to a hybrid IC. "Base Metal Thick Film Materials A Review of Their Technology & Applications" by Christopher R. S. Needes, E. I. Du Pont de Nemours & Co., Inc., IMC 1982 describes that a first conductor (Pd/Ag) is fired in air and subsequently a second conductor (Cu) is fired in nitrogen so that a Cu system is available.